Tag Archives: encoding

Thinking about frame rates.

Once upon a time it was really simple. We made TV programmes and videos that would only ever be seen on TV screens. If you lived and worked in a PAL area you would produce programmes at 25fps. If you lived in an NTSC area, most likely 30fps. But today it’s not that simple. For a start the internet allows us to distribute our content globally, across borders. In addition PAL and NTSC only really apply to standard definition television as they are the way the SD signal is broadcast with a PAL frame being larger than an NTSC one and both use non-square pixels. With HD Pal and NTSC does not exist, both are 1280×720 or 1920×1080 and both use square pixels, the only difference between HD in a 50hz country and a 60hz country is the frame rate.

Today with HD we have many different frame rates to choose from. For film like motion we can use 23.98fps or 24fps. For fluid smooth motion we can use 50fps or 60fps. In between sits the familiar 25fps and 30fps (29.97fps) frame rates. Then there is also the option of using interlace or progressive scan. Which do you choose?

If you are producing a show for a broadcaster then normally the broadcaster will tell you which frame rate they need. But what about the rest of us?

There is no single “right” frame rate to use. A lot will depend on your particular application, but there are some things worth considering.

If you are producing content that will be viewed via the internet then you probably want to steer clear of interlace. Most modern TV’s and all computer monitors use progressive scan and the motion in interlaced content does not look good on progressive TVs and monitors. In addition most computer monitors run by default at 60hz. If you show content shot at 25fps or 50fps on a 60hz monitor it will stutter slightly as the computer repeats an uneven number of  frames to make 25fps fit into 60Hz. So you might want to think about shooting at 30fps or 60fps for smoother less stuttery motion.

24fps or 23.98fps will also stutter slightly on a 60hz computer screen, but the stutter is very even as 1 frame gets repeated in every 4 frames shown.  This is very similar to the “pull-up” that gets added to 24fps movies when shown on 30fps television, so it’s a kind of motion that many viewers are used to seeing anyway. Because it’s a regular stutter pattern it tends to be less noticeable in the irregular conversion from 25fps to 60hz. 25 just doesn’t fit into 60 in a nice even manner. Which brings me to another consideration – If you are looking for a one fits all standard then 24 or 23.98fps might be a wise choice. It works reasonably well via the internet on 60hz monitors. It can easily be converted to 30fps (29.97fps) using the pull-up for television and it’s not too difficult to convert to 25fps simply by speeding it up by 4% (many feature films are shown in 25fps countries simply by being sped up and a pitch shift added to the audio).

So, even if you live and work in a 25fps (Pal) area, depending on how your content will be distributed you might actually want to consider 24, 30 or 60fps for your productions. 25fps or 50fps looks great on a 50hz TV, but with the majority of non broadcast content being viewed on computers, laptops and tablets 24/30/60fps may be a better choice.

What about the “film look”? Well I think it’s obvious to say that 24p or 23.98p will be as close as you can get to the typical cadence and motion seen in most movies. But 25p also looks more or less the same. Even 30p has a hint of the judder that we see in a 24p movie, but 30p is a little smoother. 50p and 60p will give very smooth motion, so if you shoot sports or fast action and you want it to be smooth you may need to use 50/60p. But 50/60p files will be twice the size of 24/25 and 30p files in most cases, so then storage and streaming bandwidth have to be considered. It’s much easier to stream 24p than 60p.

For almost all of the things that I do I shoot at 23.98p, even though I live in a 50hz country. I find this gives me the best overall compatibility. It also means I have the smallest files sizes and the clips will normally stream pretty well. One day I will probably need to consider shooting everything at 60fps, but that seems to be some way off for now, HDR and higher resolutions seem to be what people want right now rather than higher frame rates.

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XAVC Class 480 – What is it?

Has anyone else noticed that Adobe now include XAVC Class 480 in the codec options for exporting XAVC from Adobe Premiere via Media Encoder?

In case you don’t know what it is, class 480 is the 480Mb/s version of 4K XAVC. This gives a bit rate of…… drum roll……. 480Mb/s at 24/25/30fps. At 50 and 60fps it runs at a whopping 960Mb/s, this is the top limit for XAVC in it’s current form.

Sony’s PMW-F5/F55 and the FS7 currently record XAVC using Class 300 which is up to 300Mb/s at 24/25/30fps or 600Mb/s at 50/60p. So as you can see Class 480 has the potential to improve the compressed image quality from these cameras still further should it ever make it into a camera. Looking at what the cameras can currently do I’m not sure that this is possible with existing SxS or XQD media. 960Mb/s is the same data rate as Sonys 16 bit 24fps raw and it’s not possible to record that to SxS cards.

Coming back to Premier: Some interesting things happen in Premiere if you try to export an XAVC originated project out using Class 480. If the clips in your timeline are not adjusted in any way, in other words; exactly as shot, then if you export and choose Class 480 nothing happens to the footage. Premiere will “smart” export them exactly as they are as Class 300. This means that there will be absolutely no loss of image quality as the clip is not re-compressed. However if the original clip has been adjusted, for example graded, re-sized, a caption added etc then the clip will be encoded at 480Mb/s. From what I can tell where you have a mix of treated and untreated clips in a project Premiere is smart enough to pass through the untreated clips while bumping up any treated clips to 480.

Class 480 is seen as a mastering format by Sony. The extra data and lower compression makes it particularly suited to HDR productions.